Dissecting Light Sensing and Metabolic Pathways on the Millimeter Scale in High-Altitude Modern Stromatolites

Abstract

Modern non-lithifying stromatolites (STs) on the shore of the volcanic lake Socompa in the Puna are affected by several extreme conditions. Although STs were proposed as ecologic models for understanding stress response and resilience in microbial ecosystems constituting a window into the past, our knowledge of ST function is still nascent. The present study assesses for the first time light utilization and functional metabolic stratification of STs on a millimeter scale through shotgun metagenomics. In addition, a scanning-electron-microscopy approach was used to explore the community. Our results demonstrated that Bacteroidetes and Cyanobacteria play major roles as ST builders and primary producers to sustain a diverse community of heterotrophs. STs manifest a high occurrence of genes for the synthesis of UV-protecting pigments, the cryptochrome-photolyase family (CPF), and rhodopsins in the surface layers. Three different ecologic niches involving the use of light in energy production were defined. Calvin-Benson and Wood-Ljungdahl pathways were proposed as the main mechanisms for carbon fixation. Several genes account for the microelectrode chemical data and pigment measurements performed in previous publications. We also provide here an explanation for the vertical microbial mobility within the ST described previously. Finally, our study points to STs as ideal modern analogues of ancient STs.